Holdup alarm system

ABSTRACT

A holdup alarm system in which the premises to be protected are coupled to a central station via a transmission line, the central station having a source of direct current potential, a break indicator and a ground indicator connected in series with the line. The protected premises is provided with a normally closed loop circuit connected to the transmission line. The loop circuit includes a manually operable holdup switch having break contacts normally included in the loop circuit and connected to the high side of the transmission line. The holdup switch also has make contacts arranged, upon operation of the holdup switch, to be connected to the low side of the transmission line. An electronic switch is coupled between the high and low sides of the transmission line when the holdup switch is operated. An astable multivibrator is energized when the holdup switch is operated and operates the electronic switch to successively open and ground the transmission line at a repetition rate controlled by the multivibrator. Means also are provided to prevent a holdup alarm from being given upon an accidental break or ground in the wiring. Means also are provided to supervise the line and associated wiring against open and ground faults.

United States Patent pnorec'rzo enzmszs [72] Inventors Manfr d W- M l tr Primary Examiner-Thomas B. Habecker LMIIGSIM, -Jd Attorney-Charles B.Smith Frederick G. Hill, Yonkers, N.Y.; Anthony C. Martina, lr.,Ridgewood, N.Y. I PP 719,145 ABSTRACT: A holdup alarm system in whichthe premises to {221 Ffled 1968 be protected are coupled to a centralstation via a transmission [45] f' June line, the central station havinga source of direct current [73] Assgnee America! 0m Telegraph Companypotential, a break indicator and a ground indicator connected Jersey inseries with the line. The protected premises is provided with a normallyclosed loop circuit connected to the transmission {54] HOLD, ALARMSYSTEM line. The loop circuit includes a manually operable holdup switchhaving break contacts normally included in the loop 9Claiins, 1 DrawingFig.

circuit and connected to the high side of the transmission line. U.S.Cl340/276, The holdup witch also has make contacts arranged upon 340/409operation of the holdup switch, to be connected to the low [51] Int. Cl(1118b 29/00, side f the transmission line An electronic switch iscoupled G081! 19/00 between the high and low sides of the transmissionline when [50] Field of Search 340/409, the holdu witch is operated. Anastable multivlbrator is 276 energized when the holdup switch isoperated and operates the electronic switch to successively open andground the [56] References Cited transmission line at a re etition ratecontrolled by the mul- P UNITED STATES PATENTS tivibrator. Means alsoare provided to prevent a holdup alarm 3,010,100 11/1961 Muehter 340/409X from being given upon an accidental break or ground in the 3,174,1433/1965 Akin 340/409 wiring. Means also are provided to supervise theline and as- 3,456,251 7/ [969 Smith et al IMO/213.1 sociated wiringagainst open and ground faults.

I n m n m '7 i 010 m r I- J:-:

1 I7 I lo TRANSMISSION l A A I I 2 LINE i FLI i l I; l R3 E R4? as? as;117% l l l l i C4 "\I I RI R2 1 D1 E Ti 1 a c2 1 ii i c5 I i i 02 i 1- ll 4../ i i .4 I: i l i I c3 l i I q l y l l l l l Q3 3 1 .---2l 5 R6 R9l E I l i D l 1 CENTRAL I "T3I&"w i1 l5 l l a I HZ-M I l l Hi-M 22. I iD4 i I? H2 1 f l I HOLDUP 1 I DEVICES i I IIIOLDUP ALARM SYSTEMBACKGROUND The present invention relates to electrical protectionsystems and more particularly to such systems in which manually operablemeans are provided at a protected premises to signal to a centralstation the occurrence of a holdup.

As used in the electrical protection field, the term central station"usually refers to a particular type of central signal receiving station.As used herein, however, the term is intended to have a somewhat broadermeaning so as to include also other distant places where signals can bereceived and acted upon, e.g., guard stations and police stations. Theterm holdup" usually refers to an armed robbery and presents aprotection situation which must be responded to with extreme speed, asdistinguished, for example, from the usual burglary which normally willrequire more time to consummate and which, while requiring promptaction, is not usually as urgent as a holdup. However, the term holdup"is intended to include generally those emergency situations requiringextremely rapid response.

It is common for mercantile and industrial establishments to be providedwith protection devices which will be actuated upon entry of an intruderautomatically to signal the fact of such entry to a central stationwhere an operator will take the necessary action, e.g., dispatchingarmed guards or police to the premises so attacked When the premises areopen for business, some or all of the protection devices usually will bedisabled since it is not desired that innocent entry or exit besignalled. However, the protected premises are nevertheless connected tothe central station over the transmission line and, even if no entryprotection devices are operative, supervisory current will flow over theline to protect against opens on the line.

Where there is a danger of armed robber, i.e., holdup, of a protectedpremises, it is customary to have manually operable switches, usuallyconcealed, which can be operated to signal the occurrence of a holdup sothat prompt corrective action can be taken by police or otherappropriate agencies. Because of the extreme emergency represented by aholdup situation, it has been customary to make the holdup alarm signalwhich is transmitted to the central station distinctive, i.e., readilydistinguishable from the usual break and ground signals.

Many thousands of holdup alarm systems have been installed as part ofelectrical protection systems. A particularly useful example used formany years has involved the use of a manually operable switch with bothbreak and make contacts and a relay with a weighted armature and breakcontacts connected in series with the relay coil and the switch makecontacts. The supervisory current flows through the switch breakcontacts until interrupted by operation of the switch. The consequentcutting off of supervisory current actuates the central station breakdrop. Closing of the switch make contacts energizes the relay and alsorestores the flow of supervisory current, but through the relay coilrather than the switch break contacts. Energization of the relay opensits break contacts, again deenergizing the relay and cutting offsupervisory current. The relay will then continue to be successivelyenergized and deenergized at a rate dependent on the armature mechanicalconstants, which typically might involve 3 operating cycles persecond. Aspecial relay or special contacts on the break drop'at the centralstation respond to the interrupted current flow to actuate a holdupsignalling device at the central station.

While holdup alarm systems of the character described have affordedhighly satisfactory service, they lack the extreme reliability andtrouble free operation which can be obtained with properly designedsolid state circuitry.

The principal object of the present invention has been the provision ofa novel and improved holdup alarm system.

More particularly, it has been an object of the invention to providesuch a system which is extremely reliable and trouble free and whichrequires a minimum of maintenance.

Still another object of the invention has been the provision of such asystem which will not transmit a spurious holdup alarm signal in theevent of an accidental break or ground.

Another object of the invention has been the provision of such a systemwhich affords positive supervision against accidental opens or groundson the transmission line.

Other and further objects, features and advantages of the invention willappear more fully from the following description of the invention.

SUMMARY OF THE INVENTION In accordance with the invention there isprovided a holdup alarm apparatus for use in an electrical protectionsystem at a protected premises, the system having a local protectioncircuit and being coupled to a central station over a transmission line,the central station having a current sensitive break indicator, acurrent sensitive ground indicator and a source of direct currentpotential coupled in series between the high and low sides of thetransmission line. The holdup alarm apparatus at the protected premisescomprises a manually operable switch having break contactsinterconnecting the high side of the transmission line and one end ofthe local protection circuit, the manually operable switch also havingmake contacts. The other side of the local protection circuit isconnected to the low side of the transmission line. An electronicswitching device is arranged to be coupled between the high and lowsides of the transmission lines through the make contacts when thelatter are closed. An astable multivibrator is operative'ly coupled tothe electronic switch alternately to open and close the electronicswitch thereby alternately to intercouple and uncouple the high and lowsides of the transmission line at a rate determined by the repetitionrate of the multivibrator. The break contacts of the manually operableswitch are arranged normally to render the multivibrator unoperated.Means including the make contacts of the manually operable switch, whenclosed, render the multivibrator operative whereby the electronic switchis operated alternately to cause current to flow in the transmissionline and to break the transmission line thereby to subject the breakindicator at the central station alternately to periods of current flowand periods of no current flow as a signal indication of operation ofthe manually operable switch.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described inconnection with the appended drawing which illustrates an electricalprotection system embodying the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Referring now tothe drawing, there are shown a central station 10, a protected premises11 and a transmission line 12 intercoupling the central station andprotected premises. The transmission line is shown as a two conductorline which might typically be a leased telephone line. However, a singleconductor line with ground return can be used in place of a twoconductor line, as is well known in the art.

The central station 10 is provided with a break drop 13, a ground drop14 and a central station battery 15, all connected in series between thehigh and low sides of the transmission line. The term high side isintended to refer to the high potential side while the term low side isintended to refer to the low potential or ground side of the line. Thesystem shown is intended to operate with a positive potential on thehigh side and a grounded negative, but as will be explained below, apositive grounded source can be used with appropriate changes.

The break drop 13 is a current sensitive device which responds to a dropin line current below a predetermined value to give a break signal. Theground drop 14 is a current sensitive device which responds to anincrease in line current above another predetermined value to give aground signal. Such devices are well known in the art in the form ofrelays and in electronic form. in addition to responding to a decreasein line current, the break drop should respond also to alternate periodsof current flow above the break level and below the break level,commonly called a police call signal, to actuate an alarm device at thecentral station. Typically the repetition rate or frequency used for apolice call" signal is of the order of 3 cycles per second. Ifgalvanometer-type drops are used at the central station, as is common,it will be desirable to use a separate break drop relay for the "policecall" signal.

In accordance with a further aspect of the invention, a tone responsivedevice 16 may be included in series with the central station drop for apurpose to be described below.

At the protected premises the high side of the transmission line 12 isconnected through a current limiting resistor 17 and a path designatedABC-DE to a conventional burglar alarm circuit 18, the other end of theburglar alarm circuit 18 being connected by a conductor 19 to the lowside of the transmission line. A tone generator 20 is shown connected inseries between point E and circuit 18. The purpose of tone generator 20will be described below. The tone generator may be omitted and point Bconnected directly to circuit 18.

The burglar alarm circuit 18 may be of any type adapted to be connectedin a series circuit between the high side and the low side of atransmission line. Typically the circuit 18 might include a control set,door and window contacts, foil, photoelectric beams, sound responsivesystems, and capacitance responsive systems. The control set willusually provide for a day" protection and a night" protection condition.In the day protection condition some or all of the protective deviceswill be shunted and a normal day supervisory current will flow throughline 12. In the night protection condition all of the protective deviceswill be connected in the circuit and a normal "night" current will flowthrough line 12. Adjustable resistors are provided in the control set topermit adjustment of the day" and night values of current.

As is well known in the art, an alarm may be transmitted by opening thecurrent path, thereby decreasing the current flow through line 12, bygrounding the current path, i.e., shunting all following elements andthereby increasing the current flow through line 12, or by opening andthen grounding the current path. Distributed constants of the line mayprovide a current flow even with an open circuit at the protectedpremises.

For the purpose of understanding the operation of the invention, theburglar alarm circuit 18 could be considered as a resistor selected toprovide a normal value of current flow in line 12.

The path A-B-C-D-E comprises a conductor 21, break contacts Hl-B andHZ-B of manually operable holdup switches H1 and H2, respectively, aconductor 22 and diodes D6, D7 and D8. The return path includes tonegenerator 20 (if provided), burglar alarm circuit 18 and conductor 19 tothe low side of the central station line. The conductor 19 is connectedto the fixed terminals of make contacts H1M and H2-M of holdup switchesH1 and H2, respectively.

The holdup alarm switches H1 and H2 are of a type commonly used inholdup alarm systems and comprise an armature nonnally made with a fixedcontact and together forming the break" contacts, the armature beingarranged when the switch is manually operated to leave the fixed contactof the break" contacts and make with another fixed contact which,together with the armature, forms the "make" contacts. The holdup alarmswitches usually will be hidden and will be placed where they can beoperated surreptitiously by a hand, arm, foot or other part ofa body.After operation the switches preferably lock into their make positions.Any number of holdup alarm switches may be provided, e.g., at eachtellers position in a bank. The break contacts of the switches are allconnected in series. The fixed terminal of each of the make contacts isconnected to the conductor 19.

When one of the holdup alarm switches is operated, it is desiredalternately to increase and decrease the transmission line current at afixed repetition rate, e.g., 3 cycles per second, which will actuate thebreak drop at the central station to produce a holdup alarm at thecentral station. For this purpose there are provided an electronicswitching device, comprising a transistor 01, and an astablemultivibrator 23, comprising transistors Q2 and Q3.

The transistor O], which might be of the 2N4036 type, has its emitterconnected to point A and its collector connected through a conductor 24to one terminal of a Zener diode D9, which might be of the 1N4736 (6.8v.) type and to the positive side of a capacitor C5. The other terminalof diode D9 and the other side of capacitor C5 are connected toconductor 22. A resistor R1 intercouples the base and emitter oftransistor Q1. A resistor R2, a capacitor C4 and a resistor R3, inseries circuit, intercouple the base and collector of transistor Q1.

Transistors Q2 and 03 might be of the 2N2405 and 2N3 391 types,respectively. The emitters of transistors 02 and Q3 are connected toconductor 22. The collector of transistor O2 is connected to thejunctionof resistor R2 and capacitor C4 and is coupled to conductor 24 throughdiode D1 and resistor R4. Diode D1, and also diodes D2-D8, might be ofthe lN2070 type. The collector of transistor Q2 is also coupled to theemitter thereof through a capacitor C3. A capacitor C1, poled as shown,intercouples the junction of diode D1 and resistor R4 to the junction ofresistor R6 and diode D3. Resistor R6, diode D3 and a resistor R9 arecoupled in series between conductors 24 and 22.

A resistor R5, a diode D2 and a resistor R8 are connected in seriesbetween conductors 24 and 22. The base of transistor O2 is connected tothe junction of diode D2 and resistor R8 and, through series connecteddiodes D4 and D5, to point E.

The collector of transistor O3 is coupled to conductor 24 through aresistor R7 and, through a capacitor C2, to the junction of resistor R5and diode D2. The base of transistor O3 is connected to thejunction ofdiode D3 and resistor R9.

A Zener diode D10, which might be of the 1N4762 (82 v.) type is coupledbetween points A and E to limit the voltage which may appear across theelectronic components, e.g., to 82 volts, thus affording protectionagainst surges. in line current.

With the circuit in the condition shown, the metallic link A-B-C-B-Dcreates a shunt path between points A and D.-

Hence there is no operating potential for the transistors and all willbe nonconductive. Similarly all capacitors will be discharged.

When occurrence of a holdup is to be signalled, the link A-BCBD will bebroken by operation of switch H1 or switch H2 (or both) and theconsequent opening ofbreak contacts Hl-B or H2-B. as the case may be.

With the shunt link A-B C D broken current will flow from point A topoint E through the parallel combination of the emitter-base junction oftransistor Q1 and resistor R1 and the following three circuits:

a. resistor R2, capacitor C3, conductor 22 and diodes D6,

D7 and D8; b. resistor R2, capacitor C4, resistor R3, resistor R5, diodeD2, and diodes D4 and D5;

c. resistor R2, capacitor C4, resistor R3, capacitor C5, and

diodes D6, D7 and D8.

The voltage which appears across diodes D6, D7, D8 reverse biases thebase of transistor 02 through diodes D4 and D5 and prevents it frombecoming conductive. This feature prevents the multivibrator Q2, 03 fromstarting and producing a false holdup alarm signal in the event of anaccidental break in the line A-B-C-D which connects the various holdupswitches. An accidental ground on line A-B-C-D cannot cause a falseholdup signal because the electronic circuit would still be shunted byline A-B-C-D. The holdup alarm signal can be initiated only by theproper sequence of a break followed by a ground on the line A-BC-D, aswill be described.

The initial current flow through the three paths identified above issmall and will result in only a minor charging of capacitors C3 and C4and a very minor charging of capacitor C5.

When the armature of the operated holdup switch makes with the oppositecontact, i.e., when contacts Hl-M or H2-M are closed, ground potentialis applied to point D. A larger current now flows through the threepaths enumerated above, charging the capacitors C3, C4 and, byincreasing the base current through transistor 02, causing theemitter-collector junction of that transistor to become partiallyconductive and increasing the base current and consequently thecollector current of O1 to allow capacitor C5 to start charging at anappreciable rate. Simultaneously, the application of ground potential atpoint D shunts diodes D6, D7, D8 thereby removing the reverse bias fromthe emitter of transistor 02. Since the shunt provided by diodes D4, D5and the burglar alarm circuit 18 across .the base-emitter circuit oftransistor O2 is not sufficient to prevent transistor 02 from becomingconductive, the multivibrator will be ready to start when capacitor C5is sufficiently charged.

The multivibrator, as indicated on the drawing, comprises transistors02, Q3 and their associated capacitors and resistors. As is the casewith any astable multivibrator, either transistor may become conductivefirst depending on such indeterminate factors as transient circuitconditions, component tolerances, etc. Assume therefore that transistor02 becomes conductive first when capacitor C5 is sufficiently charged topermit base current to flow to transistor Q2 via the path resistor R5and diode D2 and via the path resistor R7, capacitor C2 and diode D2 andcapacitors Cl and C2 are charged in the polarities shown in the drawing.

With transistor Q2 conductive, the collector-emitter circuit thereofallows sufficient current to flow through the emitterbase junction oftransistor O1 to cause it to become fully conductive. The increasedcurrent flow through the emitter-collector circuit of transistor Q1completes the charging of capacitor C5. Current also flows through thepath consisting of resistor R6, dischargingcapacitor Cl, diode D1 andthe collector-emitter junction of transistor 02 to ground at point D.The voltage across capacitor C1 reverse biases the base of transistor Q3keeping that transistor nonconductive until capacitor C1 discharges tothe point where transistor 03 is no longer reverse biased. Transistor 03then begins to conduct, drawing base current via resistor R6 and diodeD3, and current flows through the path resistor R5, dischargingcapacitor C2 and the collector-emitter junction of transistor 03 toground at point D. The voltage drop across capacitor C2 reverse biasestransistor Q2 causing it to become nonconductive for the moment.

When transistor 02 becomes nonconductive, the base current path fortransistor O1 is removed (capacitors C3 and C4 are charged by now) andtransistor 01 becomes nonconductive. With transistor Q1 nonconductiveand the line A-B-CD open at the operated contacts Hll-B or HZ-B, thetransmission line to the central station is open and this conditionrepresents the beginning of a break period in the pulsing holdup alarmsignal.

Transistor Q2 remains reverse biased until capacitor C2 becomessufficiently discharged to overcome the bias and transistor 02 conductsagain. Because transistor 01 is now not conducting, base current fortransistor O2 is supplied from capacitor C5 (whose function is toprovide current for the multivibrator when transistor 01 isnonconductive) via the path resistor R7, capacitor C2 and diode D2. Withtransistor 02 conductive, transistor 01 is switched on again and currentflows from point A to recharge capacitor C5 and to ground via thebreakdown of Zener diode D9. The Zener diode D9 also performs thefunctions of establishing a constant voltage for the multivibrator andlimiting the charge on capacitor C5. Diode D9 might be ofthe 1N4736 (6.8v.) type.

The switching on of transistor Q1 marks the end of the break period andthe beginning of the ground period in the signalling sequence. Thetransistor Q3 becomes conductive again when the charge on capacitor C1overcomes the reverse bias voltage across resistor R6 and themultivibrator continues with transistors 02 and Q3 turning each other onand off. Whenever transistor 02 is conductive, transistor 01 isconductive and a ground is applied to the transmission line 12. Whentransistor 02 is nonconductive, transistor 01 is nonconductive and abreak appears in the transmission line 112.

Had transistor Q3 become conductive first when the multivibratorstarted, the sequence of events would have been the same, but shiftedhalf a cycle in time. The system will continue to operate at a nominalrepetition rate as determined by the time constant of the multivibratoruntil the actuated holdup switch is reset thus restoring the shuntA-B-C-D. The capacitor charges will then bleed off through theirassociated resistors and the system will return to the normal condition.It is desirable to make the repetition rate about 3 cycles per second soas to conform to the usual practice and to permit operation withexisting central station equipment.

The Zener diode D10 provides a bypass from point A to point E to permithigh voltage reverse polarity testing of the regular protection circuit,as is customary, and to dispose of any high voltage transients which mayappear on the transmission line.

Local annunciation of a holdup alarm may be provided if desired, e.g.,by connecting a diode, a local battery and an alarm bell or lamp inseries between conductors 19 and 22, the diode being poled to preventline current from flowing through the alarm device when none of theholdup switches are operated.

in a typical system the central station battery may supply a voltage inthe range of 52 to 78 volts. Typical values for the various resistorsand capacitors are set forth in the table below. It should beunderstood, however, that these values as well as the various voltages,transistor types and diode types set forth herein are given only by wayof example and should not be taken as limiting the scope of theinvention.

Resistor Ohms R1 100,000 R2 2,700 R3 1,000,000 R4 18,000 R5 12,000 R656,000 R7 3,000 R8 120,000 R8 120,000 Capacitor Microfarads C1 5.6 (35v.) C2 22.0 (15 v.) C3 0.1 v.) C4 0.5 (200 v.) C5 200.0(12 v.)

Should it be desired to operate the system with a positive groundedcentral station battery, PNP transistor Q1 would be replaced with an NPNtransistor, e.g., of the 2N2405 type. NPN transistors 02 and 03 would bereplaced with PNP transistors, e.g., of the 2N4036 and 2N3906 types,respectively. The polarities of the diodes and capacitors would bereversed and the values of resistors R2, R4, R5, R6 and R7 would bechanged to 1,800 ohms, 22,000 ohms, 18,000 ohms, 68,000 ohms and 5,600ohms, respectively.

In order to supervise the system against opens and grounds on thetransmission line and on the shunt link A-B-C-D, a tone generator 20 isshown with its output coil 25 connected in series between point E andburglar alarm circuit 18. The tone generator superimposes an alternatingsignal, e.g., 1,000 cycles per second, on the normal DC current flow. Anopen or ground on the transmission line or on the link A-BCD willprevent the tone from reaching the central station. Presence or absenceof the tone is detected by tone responsive device 16 at the centralstation, and absence of the tone is used to signal a fault condition. Ifit should be desired to supervise only the transmission line, the tonegenerator 20 may be connected between the high side and the transmissionline and resistor 17 or between resistor 17 and point A.

While the invention has been described in connection with specificembodiments thereof and in a specific use, various modifications thereofwill occur to those skilled in the art without departing from the spiritand scope of the invention as set forth in the appended claims.

What we claim is:

1. Holdup alarm apparatus for use in an electrical protection system ata protected premises, said system having a local protection circuit andbeing coupled to a central station over a transmission line having ahigh side and a low side, the central station having a current sensitiveline break indicator and a source of direct current potential coupled inseries between the high and low sides of the transmission line, theholdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting thehigh side of said transmission line and one end of said local protectioncircuit and having make contacts;

b. means interconnecting the other side of said local protection circuitand the low side of said transmission line;

. an electronic switching device;

. means including said make contacts coupling said electronic switchingdevice between the high and low sides of said transmission line whensaid manually operable switch is operated; and

e. an astable multivibrator operatively coupled to said electronicswitch alternately to open and close said electronic switch therebyalternately to intercouple and uncouple the high and low sides of saidtransmission line at a rate detennined by the repetition rate of saidmultivibrator alternately to cause current to flow in said transmissionline and to break said transmission line thereby to subject said breakindicator at said central station alternately to periods of current flowand periods of reduced current flow as a signal indication of operationof said manually operable switch.

2. Holdup alarm apparatus for use in an electrical protection system ata protected premises, said system having a local protection circuit andbeing coupled to a central station over a transmission line having ahigh side and a low side, the central station having a current sensitivebreak indicator and a source of direct current potential coupled inseries between the high and low sides of the transmission line, theholdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting thehigh side of said transmission line and one end of said local protectioncircuit and having make contacts;

b. means interconnecting the other side of said local protection circuitand the low side of said transmission line;

c. an electronic switching device;

d. means including said make contacts coupling said electronic switchingdevice between the high and low sides of said transmission line whensaid manually operable switch is operated;

. an astable multivibrator operatively coupled to said electronic switchalternately to open and close said electronic switch thereby alternatelyto intercouple and uncouple the high and low sides of said transmissionline at a rate detennined by the repetition rate of said multivibrator;

. means including said break contacts, when closed, to

render said multivibrator unoperated; and

g. means including said make contacts, when closed, to

render said multivibrator operative whereby said electronic switch isoperated, alternately to cause current to flow in said transmission lineand to break said transmis-- sion line thereby to subject said breakindicator at said central station alternately to periods of current flowand periods of reduced current flow as a signal indication of operationof said manually operable switch.

3. Holdup alarm apparatus as set forth in claim 2 in which saidelectronic switch comprises a transistor having a first electrodecoupled to the high side of said transmission line, a second electrodecoupled through said make contacts to the low side of said transmissionline and a third electrode coupled to said multivibrator whereby achange in condition of said multivibrator changes the conductivecondition of the circuit including said first and second electrodes.

4. Holdup alarm apparatus as set forth in claim 3 in which a Zener diodeis intercoupled between said second electrode and said make contacts,said Zener diode being poled to break down when said transistor isconductive between said first and second electrodes.

5. Holdup alarm apparatus as set forth in claim 4 in which a capacitoris coupled in parallel with said Zener diode and to said multivibratorto provide operating potential for the latter when said transistor isnot conductive.

6. Holdup alarm apparatus as set forth in claim 5 in which saidmultivibrator comprises second and third transistors operatively coupledbetween said second electrode of said first transistor and said makecontacts, so as to become alternately conductive and nonconductive insequence, one of said second and third transistors being arranged, whenconductive, to bias said first transistor to conduction and, when notconductive, to bias said first transistor to nonconduction.

7. Holdup alarm apparatus as set forth in claim 2 in which saidelectronic switch comprises a first transistor and said multivibratorcomprises second and third transistors, said transistors beingoperatively coupled to said transmission line to receive operatingpotential therefrom, said break contacts being arranged, when closed, toshunt said operating potential.

8. Holdup alarm apparatus as set forth in claim 7 comprising biasingmeans arranged to prevent operation of said multivibrator upon removalof said shunt, said biasing means being arranged to be shunted by saidmake contacts when the latter are closed.

9. Holdup alarm apparatus for use in an electrical protection system ata protected premises, said system having a local protection circuit andbeing coupled to a central station over a transmission line having ahigh side and a low side, the central station having a current sensitivebreak indicator, a tone responsive indicator and a source of directcurrent potential coupled in series between the high and low sides ofthe transmission line, the holdup alarm apparatus comprising:

a. a manually operable switch having break contacts interconnecting thehigh side of said transmission line and one end of said local protectioncircuit and having make contacts;

b. means interconnecting the other side of said local protection circuitand the low side ofsaid transmission line;

c. an electronic switching device;

d. means including said make contacts coupling said electronic switchingdevice between the high and low sides of said transmission line whensaid manually operable switch is operated;

e. an astable multivibrator operatively coupled to said electronicswitch alternately to open and close said electronic switch therebyalternately to intercouple and uncouple the high and low sides of saidtransmission line at a rate determined by the repetition rate of saidmultivibrator; means including said break contacts, when closed, torender said multivibrator unoperated;

g. means including said make contacts, when closed, to

render said multivibrator operative whereby said electronic switch isoperated alternately to cause current to flow in said transmission lineand to break said transmission line thereby to subject said breakindicator at said central station alternately to periods of current flowand periods of reduced current flow as a signal indication of operationof said manually operable switch; and

h. a tone generator at said protected premises and coupled in serieswith said transmission line to superimpose an alternating tone signal onthe direct current flowing in said line, absence of said tone beingdetected by said tone responsive indicator as an indication of a faultin said transmission line.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 587Dated June 22 I 1 Muehter et a1. Inventor(s) It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Col. 1, line 39, "robber" should read robbery col. 3, lines 65-66, quotemarks should appear around the word "break";

col. 4, line 45, "A-B-C-B-D" should read A-B-C-D line 51, "A-B-C-B-D"should read A-B-C-D col. 6, line 48, "R8" should read R9 Signed andsealed this 7th day of December 197? (SEAL) Attest:

EDWARD "L LFLETOHERJR. ROBERT GOTTSCHALK Attesting Officer ActingCommissionerof Patents FORM USCOMM-DC suave-ps9 U S GUVERNMENT PRINYiNGOFFICE \959 U365'3]fi

1. Holdup alarm apparatus for use in an electrical protection system ata protected premises, said system having a local protection circuit andbeing coupled to a central station over a transmission line having ahigh side and a low side, the central station having a cuRrent sensitiveline break indicator and a source of direct current potential coupled inseries between the high and low sides of the transmission line, theholdup alarm apparatus comprising: a. a manually operable switch havingbreak contacts interconnecting the high side of said transmission lineand one end of said local protection circuit and having make contacts;b. means interconnecting the other side of said local protection circuitand the low side of said transmission line; c. an electronic switchingdevice; d. means including said make contacts coupling said electronicswitching device between the high and low sides of said transmissionline when said manually operable switch is operated; and e. an astablemultivibrator operatively coupled to said electronic switch alternatelyto open and close said electronic switch thereby alternately tointercouple and uncouple the high and low sides of said transmissionline at a rate determined by the repetition rate of said multivibratoralternately to cause current to flow in said transmission line and tobreak said transmission line thereby to subject said break indicator atsaid central station alternately to periods of current flow and periodsof reduced current flow as a signal indication of operation of saidmanually operable switch.
 2. Holdup alarm apparatus for use in anelectrical protection system at a protected premises, said system havinga local protection circuit and being coupled to a central station over atransmission line having a high side and a low side, the central stationhaving a current sensitive break indicator and a source of directcurrent potential coupled in series between the high and low sides ofthe transmission line, the holdup alarm apparatus comprising: a. amanually operable switch having break contacts interconnecting the highside of said transmission line and one end of said local protectioncircuit and having make contacts; b. means interconnecting the otherside of said local protection circuit and the low side of saidtransmission line; c. an electronic switching device; d. means includingsaid make contacts coupling said electronic switching device between thehigh and low sides of said transmission line when said manually operableswitch is operated; e. an astable multivibrator operatively coupled tosaid electronic switch alternately to open and close said electronicswitch thereby alternately to intercouple and uncouple the high and lowsides of said transmission line at a rate determined by the repetitionrate of said multivibrator; f. means including said break contacts, whenclosed, to render said multivibrator unoperated; and g. means includingsaid make contacts, when closed, to render said multivibrator operativewhereby said electronic switch is operated, alternately to cause currentto flow in said transmission line and to break said transmission linethereby to subject said break indicator at said central stationalternately to periods of current flow and periods of reduced currentflow as a signal indication of operation of said manually operableswitch.
 3. Holdup alarm apparatus as set forth in claim 2 in which saidelectronic switch comprises a transistor having a first electrodecoupled to the high side of said transmission line, a second electrodecoupled through said make contacts to the low side of said transmissionline and a third electrode coupled to said multivibrator whereby achange in condition of said multivibrator changes the conductivecondition of the circuit including said first and second electrodes. 4.Holdup alarm apparatus as set forth in claim 3 in which a Zener diode isintercoupled between said second electrode and said make contacts, saidZener diode being poled to break down when said transistor is conductivebetween said first and second electrodes.
 5. Holdup alarm apparatus asset forth in claim 4 in which a capacitor is coupled in parallel withsaid Zener diode and to saiD multivibrator to provide operatingpotential for the latter when said transistor is not conductive. 6.Holdup alarm apparatus as set forth in claim 5 in which saidmultivibrator comprises second and third transistors operatively coupledbetween said second electrode of said first transistor and said makecontacts, so as to become alternately conductive and nonconductive insequence, one of said second and third transistors being arranged, whenconductive, to bias said first transistor to conduction and, when notconductive, to bias said first transistor to nonconduction.
 7. Holdupalarm apparatus as set forth in claim 2 in which said electronic switchcomprises a first transistor and said multivibrator comprises second andthird transistors, said transistors being operatively coupled to saidtransmission line to receive operating potential therefrom, said breakcontacts being arranged, when closed, to shunt said operating potential.8. Holdup alarm apparatus as set forth in claim 7 comprising biasingmeans arranged to prevent operation of said multivibrator upon removalof said shunt, said biasing means being arranged to be shunted by saidmake contacts when the latter are closed.
 9. Holdup alarm apparatus foruse in an electrical protection system at a protected premises, saidsystem having a local protection circuit and being coupled to a centralstation over a transmission line having a high side and a low side, thecentral station having a current sensitive break indicator, a toneresponsive indicator and a source of direct current potential coupled inseries between the high and low sides of the transmission line, theholdup alarm apparatus comprising: a. a manually operable switch havingbreak contacts interconnecting the high side of said transmission lineand one end of said local protection circuit and having make contacts;b. means interconnecting the other side of said local protection circuitand the low side of said transmission line; c. an electronic switchingdevice; d. means including said make contacts coupling said electronicswitching device between the high and low sides of said transmissionline when said manually operable switch is operated; e. an astablemultivibrator operatively coupled to said electronic switch alternatelyto open and close said electronic switch thereby alternately tointercouple and uncouple the high and low sides of said transmissionline at a rate determined by the repetition rate of said multivibrator;f. means including said break contacts, when closed, to render saidmultivibrator unoperated; g. means including said make contacts, whenclosed, to render said multivibrator operative whereby said electronicswitch is operated alternately to cause current to flow in saidtransmission line and to break said transmission line thereby to subjectsaid break indicator at said central station alternately to periods ofcurrent flow and periods of reduced current flow as a signal indicationof operation of said manually operable switch; and h. a tone generatorat said protected premises and coupled in series with said transmissionline to superimpose an alternating tone signal on the direct currentflowing in said line, absence of said tone being detected by said toneresponsive indicator as an indication of a fault in said transmissionline.